This invention relates generally to photoelectric smoke detectors, and more particularly, to a scatter type photoelectric smoke detector that provides an adjustable background signal.
Photoelectric smoke detectors are extensively used to provide an alarm in response to the detection of smoke particles in a protected zone. Basically, a photoelectric smoke detector includes a light source, a light responsive device for receiving light therefrom and an electrical system for producing a signal in response to a predetermined change in the light level received by the light responsive device. Two fundamental types of photoelectric smoke detectors are known and used. In the obscuration type detector, the light source and light responsive device are optically aligned across a test zone. The presence of smoke in the test zone reduces the level of light received by the light responsive device and results in the generation of an alarm signal. Conversely, the light scatter type detector employes a light responsive device that is optically shielded from a light source so as not to receive light directly therefrom. Smoke present in a test zone scatters light from the source to the light responsive device which responds to the increased level by generating an alarm signal.
Typical scatter type detectors enhance their signal to noise ratio by minimizing light leakage between a light source and a light responsive element. Maintenance of an adequate signal to noise ratio, however, has posed a problem for a number of reasons. During normal operation in the absence of smoke, substantially no light is received by the light responsive element and the detector produces no detection signal. Consequently, the amplifiers in detector circuits associated with the smoke detector tend to drift and when exposed to a smoke condition generate variable results. In addition, the signal produced by scatter detectors in response to smoke is generally very small. Externally produced noise signals imposed on a detector, therefore, can reduce significantly the systems signal to noise ratio. Accordingly, a typical scatter system will often either produce a false alarm in response to an insufficient quantity of smoke or experience saturation of its detector amplifiers making them insensitive to an actual smoke signal.
A partial solution to the above problem is disclosed in U.S. Pat. No. 4,306,230. Disclosed in that patent is a scatter type smoke detector having an orifice that transmits a limited level of light directly between a source and a light responsive element. The directly transmitted light establishes a background signal that is used to monitor the functional integrity of the detection system. A disadvantage of the disclosed detector is that the level of background light directly transmitted to the light responsive element cannot be adjusted to provide a desired signal to noise ratio. A system permitting adjustment of a background light level in a photoelectric smoke detector is disclosed in U.S. Pat. No. 3,992,102. In that device an adjustable screw disposed in the smoke detection chamber can be adjusted to vary the level of background light reflected upon a light receiving element. However, adjustment of the screw within the smoke detection chamber also varies the level of smoke reflected light received by the light responsive element thereby affecting the detectors sensitivity. In addition, certain conditions such as accumulated condensation on reflection surfaces can significantly increase the level of reflected background light received by the light responsive device and thereby cause false alarms.